1. Field of the Invention
This invention relates generally to electrical switching apparatus and, more particularly, to circuit breakers, such as, for example, arc fault circuit breakers.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Subminiature circuit breakers are used, for example, in aircraft electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. As such, they are subjected to heavy use and, therefore, must be capable of performing reliably over many operating cycles. They also must be small to accommodate the high-density layout of circuit breaker panels, which make circuit breakers for numerous circuits accessible to a user. Aircraft electrical systems usually consist of hundreds of circuit breakers, each of which is used for a circuit protection function as well as a circuit disconnection function through a push-pull handle. The push-pull handle is moved from in-to-out in order to open the load circuit. This action may be either manual or, else, automatic in the event of an overload or fault condition. If the push-pull handle is moved from out-to-in, then the load circuit is re-energized. If the load circuit had been automatically de-energized, then the out-to-in operation of the push-pull handle corresponds to a circuit breaker reset action.
Typically, subminiature circuit breakers have only provided protection against persistent overcurrents implemented by a latch triggered by a bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection. Arc faults are typically high impedance faults and can be intermittent. Nevertheless, such arc faults can result in a fire.
During sporadic arc fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to activate the automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker can add one of the elements required for sputtering arc fault protection—ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. It is still desirable, however, to provide separate indications in order to distinguish an arc fault trip from an overcurrent-induced trip.
Finally, there is an interest in providing an instantaneous trip in response to very high overcurrents such as would be drawn by a short circuit.
The challenge is to provide alternative protection and separate indications in a very small package, which will operate reliably with heavy use over a prolonged period. A device which meets all the above criteria and can be automatically assembled is desirable.
In aircraft applications, two practical considerations make automatic operation difficult to achieve and, possibly, undesirable. First, the design of a conventional aircraft circuit breaker makes it difficult to add an externally initiated tripping circuit thereto. Second, certain circuits on an aircraft are so critical that manual intervention by a crewmember may be desirable before a circuit is de-energized.
U.S. Pat. No. 6,542,056 discloses a movable and illuminable arc fault indicator including a first leg having a notch near the lower end thereof. The notch is engaged by a first arm of a spring. The spring has a central portion, which is held by a pin on a mechanism plate, and a second arm, which is held between side-by-side pins on the plate. The indicator also includes a second leg or light pipe member and an illuminable ring portion, which is connected to the legs. The indicator is normally recessed within the bezel of a circuit breaker housing. Under normal operating conditions, an arc fault circuit energizes a light emitting diode (LED). The free end of the light pipe is normally proximate the LED and normally receives light therefrom when the circuit is energized. Hence, the LED normally illuminates the light pipe and, thus, the illuminable ring portion. The illuminable ring portion is visible, in order to indicate, when lit, proper energization of the circuit. An indicator latch of a trip motor normally holds the first arm of the spring. When the trip motor is energized, the first arm disengages from an opening of the indicator latch and drives the first leg of the indicator upward, thereby driving the indicator ring upward to an arc fault trip position in which the light pipe is separated from the LED. As a result of the trip, power is removed to the circuit and the illuminable ring portion is no longer lit.
There is room for improvement in circuit breakers.